beaker-repo-update: use logging instead of printing to stderr

This lets us avoid stderr spew during tests ("Cannot retrieve repository
metdata" for rubbish OSMajors created during the test run).

Also added a --debug switch to control verbosity, bringing
beaker-repo-update more in line with other command-line tools which
follow a similar pattern.

Change-Id: I162e17eec2a468d45fdc4898ac0a909ad9acfcc5